This invention relates to a method and apparatus for coating plastic substrates by vapor deposition and, more particularly, it concerns such a method and apparatus for vapor deposition of optical coatings on the surfaces of optical substrates such as plastic lenses, filters and the like.
Out of problems associated with application of an assortment of coatings to optical substrates, a variety of techniques have evolved in an attempt to increase the quality and adherence of coatings, particularly on synthetic substrates without deleteriously affecting the substrates. For example, vapor polymerization, ion plating and ion assisted deposition techniques have been adopted for improving coating adherence through the use of energized particles.
An example of a vapor polymerization process is disclosed in U.S. Pat. No. 3,713,869 issued to Walter Geffcken et al. The process involves providing a plastic substrate with an intermediate polymerization layer and an outer hard inorganic layer. The polymerization layer is produced by subjecting a low-molecular organic vapor to a glow discharge and depositing the polymerization products on the substrate. The relatively hard inorganic layer is then applied to the polymerized layer by vapor deposition. As pointed out in the text of the Geffcken et al patent, the selection of glow discharge voltages and currents during application of the polymerization layer, as well as the thickness of that layer, are very critical to the achievement of an effective coating.
U.S. Pat. No. 3,913,520 issued to Joseph E. Berg et al discloses an ion plating process for implanting a hard film on plastic substrates by exposing a hard coat material to an electron beam so as to produce ionized molecular particles and using a relatively complex, multi-ring electro-static accelerator to direct the particles to the substrate(s). The respective rings of the accelerator are maintained at varying relatively high potentials modulated with an RF signal.
An example of an ion assisted vapor deposition process is disclosed in U.S. Pat. No. 4,599,272 issued to Hajime Ichikawa. In this process, a synthetic resin substrate is subjected to an argon plasma ion beam during vacuum evaporation of a material containing 10-30% SiO.sub.2. Ichikawa discloses that the use of an argon ion beam during coating improves the durability of the evaporated coating by the synergistic action of an ion-bonding effect but indicates that the evaporated coating is likely to peel when the amount of SiO.sub.2 is less than 10%.
Another example of an ion assisted deposition process is disclosed in "Ion Beam Processing For Coating MgF.sub.2 Onto Ambient Temperature Substrates" by Ursula Gibson and Charles Kennemore III. The process is similar to that disclosed in Ichikawa except that low energy, less than or equal to 250 eV, ions from a controllable ion gun source are used to bombard a plastic substrate during coating to increase adhesion of film without degrading the optical performance of the substrate. The Gibson et al article also mentions that precleaning the substrate by ion bombardment further improves film adhesion.
Although the above-described vapor polymerization, ion plating, and ion assisted deposition processes improve the adherence of coatings to synthetic substrates, there is a need for an improved process by which the adherence of optical coatings on synthetic optical substrates is substantially increased over that attainable using existing processes.